Various methods of producing a pig iron from an unreduced ore have been developed to this date, and a blast furnace method has been mainly employed at present. According to this blast furnace method, a raw material charged from the furnace top is sufficiently pre-heated by a high temperature gas flowing from below to above while the raw material falls, and iron oxide is indirectly reduced by a carbon monoxide (CO) at a ratio of at least 60%. To secure such an indirect reduction ratio in the blast furnace method, a raceway space is disposed in front of a tuyere, and a reducing gas of .eta.co (=CO.sub.2 /(CO+CO.sub.2))=0 is produced. To raise the temperature of the combustion gas serving as the high temperature gas described above, a blast temperature is set to a temperature of not less than 1,000.degree. C.
In a melting furnace using the iron source such as the iron-containing dust and/or the iron scrap as the principal material, however, the necessity for producing the reducing gas at the tuyere portion becomes lower. It is therefore believed efficient to utilize the combustion of a coke in front of the tuyere as means for securing the heat source for heating the changing material melting iron source.
In the case of the cupola method, for example, which is mainly directed to melt the iron source such as the iron scrap, the casting scrap, the pig iron, etc, but does not require the reducing function, the raw material and fuels are generally charged in mixture and melting of the iron source is generally carried out under the condition of .eta.co (gas utilization ratio)=40 to 50%. To accomplish such a gas composition, the cupola method uses a casting-coke which has a particle size of 100 to 150 mm, and prevents the solution loss reaction after the combustion of the coke. Because the large diameter coke for casting is expensive, however, it is believed effective to use a coke having a smaller particle diameter so as to reduce the fuel cost. In this case, however, the solution loss reaction rate as the endothermic reaction becomes greater and gas utilization ratio .eta.co of the coke drops, so that the melting calory drops and a stable operation becomes difficult to practice.
On the other hand, there are not many operation examples of a vertical furnace which use a self-reducible lump ore and the iron scrap as the main raw materials and requires the reducing function up to melting. Unlike the blast furnace, the raceway is not disposed in such a vertical furnace, and the operation is carried out by setting the blast temperature to not higher than 600.degree. C.
Goksel et al (Transactions of the American Foundrymen's Society, Vol. 85, AFS Des Plaines, III, (1977), pp. 327-332) report the experiment of a hot blast cupola using 5 wt % of a C-containing pellet at a blast temperature of 450.degree. C., but no prior art references have been found which deal with the operation of the normal temperature blast cupola or the operation of the vertical furnace when a large quantity of the C-containing pellet is used.
Japanese Unexamined Patent Publication (Kokai) No. 1-501401 discloses a pig iron producing apparatus comprising a blast furnace having a secondary tuyere and a hearth having a diameter more than that of the blast furnace and having a primary tuyere. According to this furnace, only the iron source is charged from the furnace top, and the fuel is directly added to the fuel bed existing at the junction between the blast furnace and the hearth. Therefore, because the inside of the blast furnace is an ore layer in which the fuel does not exist, the solution loss reaction due to the solid fuel does not proceed, and high efficient operation can be expected with an exhaust gas composition having a high CO.sub.2 /(CO+CO.sub.2) value. In this furnace, the self-reducible ore as the main material undergoes the reaction with the coke inside the coke bed at the hearth, and the melt reduction as the exothermic reaction occurs. However, because the endothermic reaction reexpressed by the following formula (2) occurs at the secondary tuyere portion, this heat is used for pre-heating, heating or melting of the iron source and the pig iron can be presumably obtained: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2 +67590 kcal/kmol.multidot.CO(2)
Since the fuel is not charged from the furnace top of the blast furnace but only the ore is charged, the coke inside the coke bed is consumed undesirably by carburizing the iron with the passage of the operation time when the continuous operation is carried out for a long time. As is obvious from the Fe--C--O equilibrium diagram, gas reduction from FeO to Fe does not easily proceed even in the self-reducible ore containing C when a gas composition has a high degree of oxidation of .eta.co.gtoreq.30% and a temperature is not lower than 1,000.degree. C. In consequence, melt reduction at the furnace lower portion becomes unavoidable, and the increase of the coke consumption quantity, the drop of the furnace heat or increase of blast pressure due to the increase of the molten liquid quantity are likely to develop. Further, when being softened and meted in the high temperature zone, the ore comes into contact with the furnace wall and turns into the adhesion, thereby resulting in so-called "shelving".
In addition to the problems described above, the shape of the furnace becomes complicated, and the problem of cooling the furnace body occurs at the time of scale-up. Therefore, a large scale furnace is believed difficult to implement.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 1-501401 described above does not concretely clarify the corelationship between the position of addition when the fuel is added from the junction portion between the blast furnace and the hearth and the primary tuyere. The primary tuyere is shown disposed between the adjacent fuel addition positions.
If the primary tuyere exists at the middle position between the adjacent fuel addition positions as described above in a furnace having a hearth mean diameter D.gtoreq.1.00 m, supplementation of the coke burnt at the primary tuyere portion is effected by the charge existing immediately above it. In this case, therefore, the ore falling from the upper part of the furnace replaces the burnt coke, and the fuel added is not believed to smoothly lower, so that the possibility of the operation stop is great.